The invention relates to a method for controlling a corona ignition device comprising a high-frequency generator and a resonant circuit that contains an ignition electrode.
WO 2010/011838 A1 discloses a corona ignition device with which a fuel/air mixture in a combustion chamber of an internal combustion engine can be ignited by a corona discharge produced in the combustion chamber. This corona ignition device comprises an ignition electrode inserted in an insulator. The ignition electrode forms an electric capacitor together with the insulator and a sleeve surrounding the insulator. This capacitor is part of an electric resonant circuit of the corona ignition device, said circuit being excited by a high-frequency AC voltage. A voltage excess is thus produced at the ignition electrode, such that a corona discharge is formed at said electrode.
The corona discharge should not break down into an arc discharge or spark discharge. Care is therefore taken to ensure that the voltage between the ignition electrode and ground remains below the breakdown voltage. To this end, the primary-side impedance of a high-frequency generator, which generates the high-frequency AC voltage from a primary voltage, is set to a target value. In order to operate the corona ignition device with the best target value possible, a given target value is checked at predefined intervals by being increased in steps from engine cycle to engine cycle until an arc discharge ultimately occurs. If an arc discharge is detected, the target value for subsequent engine cycles is reduced by a predefined percentage.
In order to generate the best corona discharge possible, corona discharge devices are generally operated just below the breakdown voltage. The closer the voltage at the ignition electrode is to the breakdown voltage, the greater is the corona discharge. However, an increase of the voltage to the breakdown voltage is to be avoided, since an arc discharge then forms instead of the corona discharge. Arc discharges cause increased burn-up of the ignition electrode and lead to a poorer combustion or ignition of the fuel/air mixture in the combustion chamber of the engine.
In corona ignition devices, a creeping discharge may occur instead of a corona discharge, or a spark or creeping discharge forms during a corona discharge. It is known from DE 10 2011 053 169 A1 that malfunctions of this type can be recognized on a characteristic profile of an electric variable. Specifically, periodic fluctuations of the secondary voltage often occur as a precursor of serious malfunctions, in particular internal spark or creeping discharges. If fluctuations of this type are detected, malfunctions can be identified early, before they lead to a serious malfunction.